The long-ruling CCM party ran almost entirely unopposed in local government polls on November 24.
The opposition boycotted, citing violence and intimidation that rights groups say have become a hallmark of Magufuli's rule.
His party clinched most of the 16,000 seats for street and village leaders -- influential posts essential for grassroots campaigning ahead of polls next year in which Magufuli is expected to seek re-election.
Chadema, the main opposition party, refused to take part, saying its candidates were fearful or disqualified from running by stringent rules used to block them.
Five smaller parties also joined the boycott.
Washington and London said the East African country's refusal to provide accreditation to respected election monitors ahead of the poll eroded confidence in the outcome.
"This troubling development calls into question the credibility of the election process and results," the US embassy in Tanzania said in a statement.
Britain's High Commissioner to Tanzania, Sarah Cooke, said in a statement that the "coordinated" disqualification of opposition candidates contributed toward voters being denied their right to choose their own leaders freely and fairly.
Magufuli on Tuesday congratulated his party on the victory and said the opposition's decision not to participate in the poll was an act of free will.
"Boycotting an election is also democratic," he told a rally in the northwestern Shinyanga region.
The ruling party's secretary general Bashiru Ally, speaking at a separate rally Tuesday, said the landslide win was proof "people still have confidence in the CCM".
The opposition has faced increasing hostility under Magufuli, who was elected in 2015 on promises to crack down on corruption but whose tenure has been marred by attacks on his critics and the press.
Once a beacon of stability in the region, Tanzania fell 25 places this year on Reporters Without Borders' press freedom index.
Britain and the US have raised concerns over Tanzania's trajectory in the past, pointing in particular to abuses of due process in cases involving government critics.
Erick Kabendera, a prominent journalist known for his unflinching coverage of Magufuli, was arrested in July and charged in a case that rights group say is politically motivated. A court in November postponed his trial for organised crime and financial offences for an eighth time.
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